In many cardiac ultrasound imaging systems, the transducer is a single-element crystal or probe which is mechanically scanned or rotated back and forth to cover a sector over an angle of 30 to 90 degrees. Acoustic signals are transmitted during the scanning and echoes from these acoustic signals are received to provide data representative of the density of tissue over the sector. Such ultrasound probes are typically driven by a closed loop system in which a signal representative of the probe position is compared to a command signal representative of the desired probe position to drive a motor which moves the probe. Typically, a short pulse burst is transmitted for each acoustic line. As the probe is swept through the sector, many acoustic lines are processed building up a sector-shaped image of the patient. The timing of each pulse burst is a function of the angular position of the probe, and it is desirable to fire the probe at exactly the same angular position for each acoustic line as the probe repeatedly scans through the sector. A sector image may be updated twice for each cycle of probe movement, once in each direction as the probe scans forward and then returns to the beginning position.
One problem with such systems is frame jitter in the final image. This is caused by small differences in the detected echoes between scans in opposite directions as the probe moves back and forth across the sector. Jitter manifests itself as an oscillating sideways movement of the image at a frequency equal to the probe scanning frequency. Reducing the amount of jitter improves the resolution with which objects may be detected and distinguished by an operator.
Two different factors contribute to this jitter. The first factor results from inherent delays in the electronics used to detect the position of the probe and to trigger the acoustical pulse used for imaging. Although these delays are relatively constant, the image displacement alternates between positive and negative directions as the probe reverses direction during each cycle.
A second factor contributing to jitter is a depth-dependant component. This effect results from the probe being constantly in motion between the time it transmits and receives returning echoes. Following the transmission of a pulse, the probe continutes to scan and is slightly displaced in position when the returning echoes are received This causes a displacement of the center of the scan line in the direction of the probe movement, resulting in a jitter in the final image as the probe rotation changes direction during each scan. The amount of this jitter is a function of both the probe velocity, probe beam pattern, and the depth from which the transmitter pulses are reflected back.